Over the past several decades, the U.S. Government, operators of underground mines, and universities have expended considerable effort in improving mine safety. Since the 1970's these activities have included the development of seismic monitoring systems to pinpoint localized seismic events in the mine, such as rockbursts. Similar efforts have been geared toward locating trapped miners in the event of an emergency. Both types of seismic monitoring systems are related, in that they can include interconnected geophones buried near the surface level. The rockburst system generally uses more permanently installed geophones, while the emergency system generally uses portable surface geophones which can be installed and configured in a few hours.
In general, the typically permanently installed rockburst systems apply a limited number of sensors spread out over a wide area, such as over the entire footprint of the mine that can extend for miles in several directions. This widely-spaced, permanent array can provide coarse measurements suitable for monitoring large, noisy, low frequency seismic events, such as rockbursts, and pinpointing the general location of these events in the mine. Unfortunately, the signal-to-noise ratio of smaller man-made seismic events, such as a trapped miner pounding on a roof bolt with a hammer, is much lower. Due to the unique characteristics of the rock strata overlying each mine, the rapid attenuation of the high frequency noise traveling through the rock, and the long distance between sensors, it is difficult to accurately capture these less-powerful man-made seismic vibrations. Furthermore, at present day is it prohibitively expensive to install and maintain a permanent geophone network over a mine extending tens of square miles with enough sensors to accurately pinpoint a man-made seismic signal at any random location in the mine.
In an emergency, portable systems can provide a higher resolution by placing a greater number of geophones directly over the impacted area to improve sensitivity to human-caused events. Although these types of systems are never exact, rescuers can compare the general direction of man-made impact signals generated by trapped miners with a map of the mine to determine an approximate location. The disadvantages of portable systems are obvious, however. Being portable, they must be carried to the accident site and, depending upon the surface terrain, may take hours or days to set up and configure. This is a critical shortcoming in situations where time is of the essence, with trapped miners having limited quantities of air, sustenance and heat. Furthermore, since there is no opportunity to calibrate the system to the specific rock strata overlying the mine, the location solutions are only approximate at best.
What is needed is a seismic monitoring system for locating trapped miners with an improved capability to detect man-made seismic signals without a high number of seismic sensors and where the seismic vibrations emitted by a trapped miner signal-to-noise ratio is poor.